Degradation of cyanoacrylic acid-based organic sensitizers in dye-sensitized solar cells

Article abstract of DOI:10.1002/cssc.201200949

Organic dyes have become widely used in dye-sensitized solar cells (DSSCs) because of their good performance, flexible structural modifications, and low costs. To increase the photostability of organic dye-based DSSCs, we conducted a full study on the degradation mechanism of cyanoacrylic acid-based organic sensitizers in DSSCs. The results showed that with the synergy between water and UV light, the sensitizer could desorb from the TiO₂ surface and the cyanoacrylic acid unit of the sensitizer was transformed into the aldehyde group. It was also observed that the water content had a great effect on the degradation process. Our experiments conducted using ¹⁸O-labeled water demonstrated that the oxygen atom of the aldehyde group identified in the degraded dye came from the solvent water in the DSSCs. Therefore, controlling the water content during DSSC fabrication, good sealing of cells, and filtering the UV light are crucial to produce DSSCs that are more durable and robust. Dye-ing to degrade: The degradation mechanism of cyanoacrylic acid-based organic sensitizers in dye-sensitized solar cells (DSSCs) has been studied. With the synergy of water and UV light, the sensitizer desorbs from the TiO₂ surface and the cyanoacrylic acid unit of sensitizer is converted into an aldehyde group. It is also observed that the oxygen atom of the aldehyde comes from the solvent water in DSSCs. Copyright

Full text of DOI:10.1002/cssc.201200949

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DOI: 10.1002/cssc.201200949
Degradation of Cyanoacrylic Acid-Based Organic
Sensitizers in Dye-Sensitized Solar Cells
Cheng Chen,[a] Xichuan Yang,*[a] Ming Cheng,[a] Fuguo Zhang,[a] and Licheng Sun*[a, b]
Organic dyes have become widely used in dye-sensitized solar
cells (DSSCs) because of their good performance, flexible struc-
tural modifications, and low costs. To increase the photostabili-
ty of organic dye-based DSSCs, we conducted a full study on
the degradation mechanism of cyanoacrylic acid-based organic
sensitizers in DSSCs. The results showed that with the synergy
between water and UV light, the sensitizer could desorb from
the TiO2 surface and the cyanoacrylic acid unit of the sensitizer
was transformed into the aldehyde group. It was also observed
that the water content had a great effect on the degradation
process. Our experiments conducted using 18O-labeled water
demonstrated that the oxygen atom of the aldehyde group
identified in the degraded dye came from the solvent water in
the DSSCs. Therefore, controlling the water content during
DSSC fabrication, good sealing of cells, and filtering the UV
light are crucial to produce DSSCs that are more durable and
robust.
Introduction
Dye-sensitized solar cells (DSSCs) have emerged as an alterna-
tive to inorganic silicon-based solar cells for commercialization
because of their low cost, easy fabrication, and ecofriendly spe-
cialty.[1,2] Although the price of silicon-based solar cells has
dropped, DSSCs still have a price advantage if low-cost organic
sensitizer can be utilized. At present, the biggest challenge for
the commercialization of DSSCs is device stability, especially
with organic sensitizers. There are several factors that can
affect the stability of DSSCs including the stability of the dyes,
the electrolyte, sealing the materials to UV light, water, and
heat, as well as the corrosion resistance of the sealing
materials.[3–5]
using a N719-dye-adsorbed TiO2 electrode and a carbon coun-
ter electrode.[13] However, the commercialization of DSSCs
using metal sensitizers is limited by the high price of noble
metals and the environmental concerns. Compared to noble
metal complexes, metal-free organic dyes are a good choice
because they are easier to synthesize, have higher molar ex-
tinction coefficients, and are less expensive.[14] Organic dyes in
which cyanoacrylic acid are adopted as an electron acceptor
are in general use. The highest conversion efficiency (h) value
for a cyanoacrylic acid-based organic DSSC was between
10.0% and 10.3%, which was achieved in full sun light by
Zeng et al.[15]
As the core part of the DSSCs, stable photosensitizers are
particularly important for the commercialization of DSSCs. The
degradation of noble metal sensitizers has been investigated
in detail, and were proven to be relatively stable.[6–11] From ac-
celerated ageing tests, the lifetime of industrial Z907-based
DSSCs was 40 years under middle European conditions and
25 years for southern European conditions.[12] The longest du-
rability test, of almost 2.5 years, for the DSSC module under
outdoor working conditions was successfully performed by
A large number of previous studies in our group have dem-
onstrated that organic dyes containing cyanoacrylic acid as an
electron acceptor could be easily damaged under long illumi-
nation times or at high temperatures. The stability of organic
dyes has become a new challenge for the industrialization of
DSSCs. As a result, a detailed study on the degradation of or-
ganic dyes was necessary for further progress in DSSC technol-
ogy. In this work, three types of organic dyes, each containing
a cyanoacrylic acid unit, were used to explore the degradation
mechanism of the dyes. To the best of our knowledge, there
are no previously published reports on the specific degrada-
tion mechanism of these organic dyes.
[a] Dr. C. Chen, Prof. X. Yang, Dr. M. Cheng, Dr. F. Zhang, Prof. L. Sun
State Key Laboratory of Fine Chemicals
DUT-KTH Joint Education and Research Centre on Molecular Devices
Dalian University of Technology, Dalian 116024 (PR China)
Fax: (+86)41184986250
Results and Discussion
E-mail: yangxc@dlut.edu.cn
[b] Prof. L. Sun
Degradation of cyanoacrylic acid-based organic sensitizers
School of Chemical Science and Engineering
Centre of Molecular Devices, Department of Chemistry
KTH Royal Institute of Technology, Stockholm 10044 (Sweden)
Fax: (+46)8-791-2333
To gain an insight into the degradation of organic sensitizers,
three types of organic dyes and an electrolyte containing
0.04m LiI and 0.04m I2 in acetonitrile (the solvent is without
any treatment, water content=0.083%) were used to fabricate
DSSCs under normal atmospheric conditions (the active area
E-mail: lichengs@kth.se
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201200949.
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of the cell was 6ꢁ6 cm). The chemical structures of the organic
sensitizers (TPA1, TPA2, and TPA3) are shown in Figure 1. All
of these sensitizers have the same donor group (triphenyl-
amine) and the same acceptor group (cyanoacrylic acid), but
Scheme 1. Degradation of the cyanoacrylic acid-based sensitizers to the cor-
responding aldehydes.
Herein, the factors of ubiquitous water, ambient temperature,
À
the IÀ/I3 redox couple, and UV light are discussed, considering
both the fabrication and test conditions.
Effect of water and water content
Figure 1. Molecular structures of the cyanoacrylic acid-based sensitizers:
TPA1, TPA2, and TPA3.
DSSCs are usually fabricated under normal atmospheric condi-
tions, in which a trace amount of water is always present.
Thus, the presence of water is inevitable during the fabrication
of DSSCs.[19] Extensive studies on DSSCs have shown that the
presence of water is detrimental for DSSCs efficiency and long-
term stability,[20–26] but there has been no report about the
effect of water on the degradation of organic sensitizers until
now. To obtain an insight into the effect of water in the degra-
dation of sensitizers, an electrolyte containing 0.04m LiI and
0.04m I2 in dry acetonitrile (the solvent was treated with CaH2
and distilled under N2 atmosphere, water content=0.009%),
and dyes TPA1, TPA2, and TPA3 were used to fabricate DSSCs.
The fabrication of the DSSC devices and the preparation of the
electrolyte were both performed in the glove box to avoid
water as much as possible. For comparison, other DSSC devi-
ces were fabricated with 10vol% water deliberately added to
the electrolyte. After an ageing test over 300 h, the color of
the DSSCs containing 10vol% water in the electrolyte became
much lighter than the DSSCs
with different p-bridges. TPA1 was fabricated without a
p-bridge, whereas TPA2 and TPA3 were constructed with
a phenyl bridge and a thiophene bridge, respectively. The
three sensitizers and their intermediates were synthesized ac-
cording to previously published procedures.[16–18]
After an ageing test for 300 h, the devices were dismantled
and the dyes were desorbed by using CH2Cl2. New compounds
were detected in the dye solution (see Figure 2), which were
all purified by using column chromatography. Through the
1
analysis of MS and H NMR spectra (see the Supporting Infor-
mation), it was identified that the cyanoacrylic acid unit in the
sensitizers was converted into the aldehyde group (see
Scheme 1).
According to the phenomenon, there could be many rea-
sons for the degradation of cyanoacrylic acid-based dyes.
without water, see Figure 3a.
Test results showed that, for
DSSCs without water, no new
compound
was
detected
through MS. However, for the
devices fabricated with 10vol%
water, a mass corresponding to
the new compound containing
an aldehyde group was detect-
ed. This meant that the dyes
had degraded in the presence
of water and that the cyano-
acrylic acid unit was converted
into the aldehyde group. There-
fore, we concluded that water
played a crucial role in the dye
degradation.
We also prepared DSSCs devi-
ces, containing TPA2 as the sen-
Figure 2. TLC plates for the three desorbed sensitizers under 365 nm UV light. The left spot on each plate is the
pure dye and the right spot is the desorbed dye.
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quantity of the degraded dyes and the water content in
DSSCs.
From the ageing test results (see Figure 5a), it was clear that
DSSCs with no water could keep working with 3.48% efficien-
cy, and only a slightly decrease was observed after ageing for
300 h. However, the efficiency of the DSSCs decayed much
faster with increasing water content. The devices with 0.1vol%
Figure 3. a) Photographs of DSSCs (cell area: 6ꢁ6 cm) containing the TPA1,
TPA2, and TPA3 sensitizers after the aging test without water (left) and with
10vol% water (right) in the electrolyte. b) TLC plates of the desorbed TPA2
dye after the aging test with 0.1 (left) and 10vol% water (right) in electro-
lyte; the plates were visualized under 254 nm UV light.
sitizer, with different water contents (0.1, 0.5, 1, 5, and 10vol%)
in an electrolyte containing 0.6m 1,2-dimethyl-3-propylimid-
azolium (DMPII), 0.025m LiI, 0.04m I2, 0.05m guanidinium thio-
cyanate (GuSCN), and 0.28m tert-butylpyridine (TBP) in 3-me-
thoxypropanonitrile (MPN, water content=0.076%). TLC analy-
sis (Figure 3b) and absorption spectroscopy were measured to
further investigate the relationship between the quantity of
the degraded dye and the water content. From the TLC analy-
sis, the device with 0.1vol% water content contained much
more TPA2 acid, with only a small amount of the new com-
pound containing the aldehyde group. However, for the devi-
ces containing 10vol% water, most of the sensitizer had been
converted into the corresponding aldehyde.
By desorbing the degraded dye TPA2-CHO with CH2Cl2, the
quantity of the degraded dye could be estimated by measur-
ing the absorption spectrum, see Figure 4. The absorbance of
TPA2 on the cells (6 cmꢁ6 cm) was 3.4ꢁ10À6 m on average, ac-
cording to the absorption spectrum. The quantity of the de-
Figure 5. The ageing tests of DSSCs containing TPA2 as the sensitizer and
with a) different water contents and b) with 10vol% water in the electrolyte
and with or without a UV filter. Electrolyte: 0.6m DMPII, 0.025m LiI, 0.04m I2,
0.05m GuSCN, and 0.28m TBP in MPN.
graded dyes with 0.1, 1, and 10vol% water was 4.7ꢁ10À7
,
1.4ꢁ10À6, and 2.9ꢁ10À6 m, respectively. It was observed that
by increasing the water content by an order of magnitude, the
absorption intensity of the degraded dye was almost doubled,
indicating that there was a direct relationship between the
water could still operate with a reduced efficiency (2.58%)
after ageing for 300 h, whereas the efficiency of the devices
fabricated with >0.5vol% water was attenuated to almost
zero after ageing. This meant that the water content had an
important effect on the stability of the organic sensitizer-based
DSSCs. Therefore, during DSSC fabrication, the water content
in the devices should be controlled within 0.1vol% to obtain
a higher efficiency and better durability.
Effect of ambient temperature
DSSCs were illuminated under 508C irradiation temperature for
the stability test, thus ambient temperature could play a role
for the degradation of sensitizers. Herein, dye TPA2 was dis-
solved in MPN (containing 10vol% water), and was then
heated to 1658C for 1 h. It was detected by using MS that the
cyanoacrylic acid unit of the TPA2 was converted into the alde-
Figure 4. The absorption spectra of the degraded compound (TPA2-CHO) in
CH2Cl2 from DSSC devices with different water contents.
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Scheme 2. a) The reaction of dye TPA2 in MPN (containing 10vol% water) upon heating to 165 and 508C. b) The reaction of dye TPA2 in dehydrated MPN
upon heating to 1658C and the reaction of the decarboxylated TPA2 dye in MPN (containing 10vol% water) upon heating to 1658C.
hyde group (see the Supporting Information). In contrast, dye
TPA2 in MPN (containing 10vol%) that was heated to 508C for
3 h, saw no conversion (see Scheme 2a). At the same time, it
was observed that when TPA2 dissolved in dehydrated MPN
was heated to 1658C for 1 h, a decarboxylation reaction oc-
curred (see the Supporting Information). However, when the
decarboxylated TPA2 was dissolved in MPN (containing
10vol% water), and then heated to 1658C for 3 h no further
conversion could occur, and TPA2-CHO was not formed (see
Scheme 2b). Therefore, it was deduced that the ambient tem-
perature has no effect on the degradation of sensitizers, and
the presence of the carboxylic acid group in the dyes could ac-
tivate the degradation reaction between water and the
cyanoacrylic acid group.
ticipate in the transformation process of the cyanoacrylic acid
unit to aldehyde group.
Effect of UV light
To test whether UV light plays a role in this transformation pro-
cess, DSSC devices were fabricated with and without the UV
membrane (filter lꢀ400 nm light). TPA2 was used as the sen-
sitizer, and 10vol% water was added to the electrolyte. The
corresponding data from the ageing tests are shown in Fig-
ure 5b. The initial efficiency of the device with the UV mem-
brane was slightly lower than that of the corresponding device
without the UV membrane; this was because the UV filter
could partly filter visible light. The DSSC device with the UV
membrane could operate with a relatively stable efficiency,
whereas the device without the UV membrane exhibited zero
efficiency after the ageing test. The results showed that UV
light, cooperating with water, played a crucial role in the deg-
radation of the sensitizer. The synergy of water and UV light
caused the cyanoacrylic acid unit to transform into the alde-
hyde group.
À
Effect of IÀ/I3 redox couple
Unlike the normal assembly of DSSC devices, the devices were
fabricated with only acetonitrile and 10vol% water-doped ace-
tonitrile instead of the normal electrolyte. This was used to ex-
À
amine whether the IÀ/I3 redox couple participated in the deg-
radation of the TPA2 sensitizer. It was observed that the dye of
the DSSCs degraded rapidly, probably because the dye was in
an oxidation state that could not receive electrons from the
Degradation mechanism of dyes
To further understand the transformation process of the cya-
noacrylic acid unit to the aldehyde, a TPA2-based DSSC device
containing 5vol% isotope-labeled water (D218O) was created.
D218O was added to the electrolyte containing 0.04m LiI and
0.04m I2 in acetonitrile (water content=0.009%). After the
À
electrolyte without the IÀ/I3 redox couple. However, a small
amount of the new compound containing the aldehyde group
(TPA2-CHO) was still detected (see the Supporting Informa-
À
tion). Therefore, the IÀ/I3 redox couple did not necessarily par-
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that the TiO2 itself participated
in the degradation of the dyes.
It is well known that the h+
species is
a strong oxidant,
which can oxidize D218O into
a D218O radical cation.[28] In our
case, the D218O radical cation
could attack the double bond
in the cyanoacrylic acid and
cause the dye to desorb from
the TiO2 surface in the form of
an aldehyde. In our work, the
18O atom in the aldehyde group
of the TPA-CHO compound
came from water. It was demon-
strated that neither UV light nor
water was dispensable for the
degradation of cyanoacrylic
acid-based sensitizer, but the
Figure 6. MS spectrum of the decomposed product, indicating the formation of TPA2-CH18
O
ageing test, a new compound (TPA2–CH18O, m/z=377.1666)
was detected through MS, see Figure 6. This new compound
contained an 18O atom but no D atom, which indicated that
during the transformation process, the 18O atom in the alde-
hyde group came from the water in the electrolyte.
combination of both UV light and water caused the degrada-
tion of the dyes.
Conclusions
From the comprehensive analysis of the experimental results
obtained above, the possible degradation mechanism of cya-
noacrylic acid-based organic sensitizers in DSSCs was identi-
fied, as it is shown in Figure 7. Firstly, dyes TPA1, TPA2, or
TPA3 in CH2Cl2 were adsorbed onto the TiO2 surface through
In conclusion, with the synergy of water and UV light, the sen-
sitizer of the cyanoacrylic acid-based organic DSSCs can de-
grade and desorb from the TiO2 surface. The cyanoacrylic acid
unit can be converted into the corresponding aldehyde and
the oxygen atom in the aldehyde group comes from the water
in the electrolyte of the DSSC. Moreover, the water content
plays a vital role in the degradation of the sensitizer. With in-
creasing water content, the efficiency of DSSCs decays much
faster. However, if the water content can be kept within
0.1vol% during fabrication, the DSSCs can operate with only
a slightly decreased efficiency for a prolonged time. The IÀ/I3
À
redox couple has little effect on the degradation of the sensi-
tizer. Therefore, controlling the water content during the fabri-
cation of DSSCs and filtering the UV light are crucial to pro-
duce DSSCs that are more durable and robust.
Experimental Section
Analytic measurement
1H NMR spectra were obtained by using a INOVA (Varian) 400 MHz
spectrometer with the chemical shifts measured against a tetrame-
thylsilane internal standard. MS data were obtained by using GCT
CA156 (Micromass, UK), HP1100 LC/MSD (Agilent, USA), and
LC/MALDI TOF (Waters, USA) spectrometers.
Figure 7. The possible degradation mechanism of cyanoacrylic acid-based
organic sensitizers in DSSCs. M is the molecule, which has the same donor
(triphenylamine group), but different p-bridges.
a bidentate chelation mode or a bidentate bridging mode.[17,27]
The photoinduced electron (eÀ) and photoinduced hole (h+)
couple could be generated when anatase TiO2 was irradiated
with UV light (lꢀ388 nm). An experiment was performed to
test whether TiO2 had an effect on the degradation of dyes.
It was observed that the dyes dissolved in acetonitrile (at pH=
8 to ensure that it contained an acetate bond) with 10vol%
water did not degraded under irradiation after 48 h, indicating
Fabrication of DSSCs
Two types of DSSCs were fabricated with active areas of 6ꢁ6 cm
and 1ꢁ1 cm. The method of TiO2 film fabrication is described in
the Supporting Information. The film was immersed into
a
2ꢁ10À4 m dye solution in CH2Cl2 (the solvent was treated with
CaH2 and distilled under N2 atmosphere, water content=0.005%)
and maintained under dark conditions for 12 h. The film was then
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Dupont). The electrolyte (of the DSSCs with an area of 1ꢁ1 cm)
consisting of 0.6m DMPII, 0.025m LiI, 0.04m I2, 0.05m GuSCN, and
0.28m TBP in MPN with different water contents (0, 0.1, 0.5, 1, 5,
and 10vol%) was introduced into the cell via vacuum backfilling
through the hole in the back of the counter electrode. The electro-
lyte (of the DSSCs with an area of 6ꢁ6 cm) consisting of 0.04m LiI
and 0.04m I2 in acetonitrile with different water contents (0, 0.1, 5,
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The sensitized film was immersed in CH2Cl2 for 3 h. The degraded
dyes without the cyanoacrylic acid anchor group could then easily
dissolve in CH2Cl2, and a small part of the non-degraded dye also
dissolved in the solution. The remainder of the non-degraded dye
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The ageing tests of the DSSCs were conducted under a Xenon
lamp in a weather-resistant test chamber. During the ageing tests,
the Xenon-lamp was irradiating at 1000 WmÀ2, and the tempera-
ture was 508C. The irradiation source for the photocurrent densi-
ty–voltage (J–V) measurements was an AM 1.5G solar simulator
(16S-002, SolarLight Co. Ltd., USA). The incident light intensity was
100 mWcmÀ2 calibrated with a standard Si solar cell. The tested
solar cells were masked to a working area of 0.159 cm2. The J–V
curves, used to obtain the efficiencies in the stability testing, were
obtained by using a linear sweep voltammetry (LSV) method with
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We gratefully acknowledge financial support of this work from
the China Natural Science Foundation (Grant Nos. 21076039,
21276044, 21120102036, and 20923006), the National Basic Re-
search Program of China (Grant No. 2009CB220009), the Swedish
Energy Agency, K&A Wallenberg Foundation, and the Program
for Innovative Research Team of Liaoning Province (Grant No.
LS2010042) from the State Key Laboratory of Fine Chemicals
(KF0805). Thanks to Sean Baxter for discussions.
Received: March 15, 2013
Revised: May 10, 2013
Keywords: degradation · dyes/pigments · isotopes · reaction
mechanisms · solar cells
Published online on June 14, 2013
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